i have 2 routers running tomato.
first router handles dhcp ect ect basically everything
second router i use to extend wireless no dhcp or anything

My question is how can i see what mac address is connected to LAN3 port on router #2 ? its a wired connection but the router shows nothing in the device list since its just passing the data to router 1.

My problem is I have 5 total routers in the network all passing data to the primary router and i never created a topology or labeled anything now i have no idea what is actually connected to what unless i disconnect a port and then try to find the device that's not working. I am going to remove some of the extenders from the network but it would be nice to know what is connected to where so i can get a plan together before i start removing devices.

I know i know I should have labeled my cables it just kept growing over time and i never did.

My question is how can i see what mac address is connected to LAN3 port on router #2 ?

You cannot. LAN ports work like a network switch ( which they are ) and the router cannot see the data stream unless the packets transverse the gateway. Then it is of course the gateway that will see them, not your collection of network switches.